Metal fatigue of a fan blade has been blamed for the engine failure that forced an Air Asia flight to return to Perth earlier this year, with the captain telling passengers to "say a prayer".
Flight D7237 departed for Kuala Lumpur on June 25 with 359 people on board and was about 90 minutes into the flight, near Carnarvon, when a problem was detected.
In an interim report, the Australian Transport Safety Bureau said a fan blade on the number one engine failed during ascent, causing engine damage and significant airframe vibration.
The engine manufacturer found that a section of the fan blade separated from the blade base.
"The blade section and associated engine debris were retained within the engine cowls," the ATSB said on Wednesday.
"Examination of the retained fan blade section identified that the separation was likely due to metal fatigue that originated within the blade's internal structure."
Analysis of the failure mode is ongoing and the full ATSB report is expected in November.
Passenger Sophie Nicolas said at the time of the incident that she knew the situation was "really bad" judging by the cabin crew's reaction.
"He (the captain) said 'I hope you all say a prayer. I will be saying a prayer too and let's hope we all get back home safely'," she said.